ZIP

(Zip i) is logically equivalent to (equal (ifix i) 0) and is
the preferred termination test for recursion through the integers.
(Zip i) returns t if i is 0 or not an integer; it
returns nil otherwise. Thus,

For a discussion of the various idioms for testing against 0,
see zero-test-idioms.

Zip is typically used as the termination test in recursions
through the integers. It has the advantage of ``coercing'' its
argument to an integer and hence allows the definition to be
admitted without an explicit type check in the body. Guard
verification allows zip to be compiled as a direct
=-comparision with 0.